Packet based networks were originally constructed in order to transmit data between computers. Packet based networks comprise a plurality of routers interconnected through communication links. Data transmitted in packet based networks are encapsulated in packets which each contain an address of a destination of the packet. Bach router forwards packets it receives toward the destination of the packets.
Due to the low cost of packet based networks relative to switched networks (e.g., the standard telephone network) packet based networks are now used for many additional services, such as multimedia streaming, data broadcasting and telephone and fax transmission. Most homes, however, are not connected directly to a packet based network but rather are connected to switched networks. In order to access packet based networks, users establish a modem connection over the switched telephone network to a gateway, e.g., an Internet service provider (ISP), which connects the users to the packet based network.
Cellular telephone networks provide telephone communications to telephone sets, referred to as mobile stations, which are not confined to a specific location. As with wired telephone networks, it is desired to use the mobile network to access packet based networks from the mobile stations. In order to provide data services over mobile networks efficiently, the General Packet Radio Service (GPRS) protocol was defined. The GPRS protocol is described, for example, in HTTP://www.cisco.com/warp/public/cc/so/neso/gprs/gprs_wp.htm, available on Aug. 22, 2001, and in the GSM technical specification, titled “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; GPRS; Service description; Stage 2 (Release 1997)”, of the GSM group, identified as 3GPP TS 03.60 V6.9.0 (2001-06) available for example from ftp://ftp.3gpp.org/Specs, the disclosures of which are incorporated herein by reference. According to the GPRS protocol, a mobile station (MS), which requires a data session, registers with a serving GPRS support node (SGSN). Thereafter, the MS generates a context request which initiates a context assignment procedure in which the MS is associated with a packet based address of an external packet based network.
In some cases, packet based networks are used to provide multicast services, in which a single source transmits the same data to a plurality of recipients. In order to reduce the bandwidth consumption of multicast services, multicast routing protocols, such as IGMP (Internet gateway multicast protocol), MOSPF, PIM-SM/DM and SSM, have been defined. According to these protocols, each router receives only a single copy of multicast data, and forwards one copy of the data in each direction in which there is at least one recipient desiring to receive the data.
The elements of cellular networks do not generally support multicasting. Therefore, when a gateway between a packet based network and a switched network (e.g., a GGSN) receives multicast packets it duplicates them for each mobile station requiring to receive the packets. This is very wasteful in bandwidth.
Proprietary multicast schemes for cellular networks do exist. The Cell Broadcast Service (CBS), for example, provides low bit rate multicasting to a group of mobile stations. U.S. Pat. No. 5,887,252 to Noneman, the disclosure of which is incorporated herein by reference, describes using multicast transmission over CDMA cellular networks. Noneman describes multicasting from a base station to a group of mobile stations. The Noneman patent, however, does not state a method for leading multicast packets within the terrestrial mobile network.
U.S. Pat. No. 5,978,368, which is incorporated herein by reference, suggests pre-generating unassigned channels in a terrestrial cellular network. When a mobile station requests to establish a channel, a pre-generated channel is assigned to the mobile station. This method reduces the delay between requesting the channel and receiving the channel.